If the Nazgűl were actually in any way significant presence in The Hobbit, there would be a hell to explain why they don't sense the numerous times Bilbo puts on the Ring, and go after him on the spot.

I actually hope that PJ will cut that bit about the tombs from the theatrical release, and put it in the extended cut as a little extra spice to tie the two movie series together. On its own it does very little for the story, but if the Nazgűl were actually in the story in person, they would change the plot massively with their presence.